RFR: 8349662: SSLTube SSLSubscriptionWrapper has potential races when switching subscriptions

[Daniel Fuchs]

Hi,

Please find here a change that fixes a potential race condition in  SSLTube.SSLSubscriptionWrapper.
Typically the race may get triggered if the  demand increased by request() is not exhausted by the time 
the subscription is switched by setSubscription.

Some synchronization is required to present a consistent view of the subscripton state, so that pending demand can be consistently transferred to the new the subscription.

This mostly affects HTTP/1.1 over TLS since each new exchange will cause the subscription to be switched to the new exchange. The race condition is elusive and hard to reproduce. when it occurs, it mostly causes tests to fail in jtreg timeout as the demand from upstream may not be transferred properly. 

Some additional logging has been added to the DigestEchoClient.java test class (which is used by DigestEchoClientSSL) to help diagnosability of intermittent failures in these tests.

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Commit messages:
 - 8349662: SSLTube SSLSubscriptionWrapper has potential races when switching subscriptions

Changes: https://git.openjdk.org/jdk/pull/23515/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23515&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8349662
  Stats: 147 lines in 3 files changed: 94 ins; 14 del; 39 mod
  Patch: https://git.openjdk.org/jdk/pull/23515.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23515/head:pull/23515

PR: https://git.openjdk.org/jdk/pull/23515